Poly aluminum sulphate is a relatively new product that has been extensively developed over the past few years especially for the field of water purification where it is useful as a flocculating agent.
Various processes for the preparation of poly aluminum sulphate have been developed over the years. Traditional methods follow a partial neutralization of aluminum sulphate (Alum) with hydroxyl groups from lime, caustic soda, soda ash, ammonium hydroxide or other alkali sources to a pH of approximately 3.5 to 4.3. Typically, the pH value tends to be 3.8 because aluminum hydroxide is not precipitated at or below this pH value.
Stabilizers such as phosphates, or chlorides may also be added to partially replace sulphate groups, or alternatively organic complexing agents such as sodium heptonate, citric acid, sorbitol, sodium citrate, sodium tartrate, sodium gluconate and the like may be added separately to stabilize the aqueous poly aluminum sulphate as much as possible. For a good review of the various processes that have been developed over the years for synthesizing poly aluminum sulphate, one may refer to Canadian Pat. Nos. 1,123,306, 1,203,364, 1,203,664 or 1,203,665 and to U.S. Pat. Nos. 4,284,611 and 4,536,384.
For instance, in Canadian Pat. No. 1,203,364, Alum is neutralized with sodium hydroxide to prepare an aluminum hydroxide gel according to the following reaction: EQU 4Al.sub.2 (SO.sub.4).sub.3 +24NaOH).fwdarw.8Al(OH).sub.3 +12Na.sub.2 SO.sub.4
Next, poly aluminum sulphate is prepared from this gel according to the following reaction: EQU 8Al(OH).sub.3 +4Al.sub.2 (SO.sub.4).sub.3 .fwdarw.4Al.sub.4 (OH).sub.6 (SO.sub.4).sub.3
This reaction yields a 50% basic product.
Using this method, 4 moles of poly aluminum sulphate and 12 moles of sodium sulphate by-product are obtained, thereby representing a significantly disadvantage. A further considerable disadvantage of this method is that a "filter cake" of aluminum hydroxide gel must be prepared before the desired product can be synthesized. Therefore, this supposes a two-step process from which large amounts of by-products are obtained.
Canadian Pat. No. 1,203,364 also teaches the neutralization of sodium aluminate with sulfuric acid to produce an amorphous aluminum hydroxide gel according to the following reaction: EQU 4Na.sub.2 Al.sub.2 O.sub.4 +4H.sub.2 SO.sub.4 +8H.sub.2 O.fwdarw.8Al(OH).sub.3 +4Na.sub.2 SO.sub.4.
This gel is then further reacted with Alum to produce basic poly aluminum sulphate. Then for a 50% basic product: EQU 8Al(OH).sub.3 +4Al.sub.2 (SO.sub.4).sub.3 .fwdarw.4Al.sub.4 (OH).sub.6 (SO.sub.4).sub.3
we note that although most of the sodium sulphate is removed from the aluminum hydroxide gel through filtration, it remains nevertheless a wasted and costly by-product. Again, this is a two-step process which is difficult to perform because one must also go through the production of a "filter cake" of aluminum hydroxide gel. This yields 4 moles of by-product per 4 moles of product.
The processes described in the remaining above-mentioned patents, which do not require a prior hydroxide gel formation, still present serious drawbacks. For example, with these processes, it is difficult to produce a concentrated marketable product containing the required typical 7 to 10% of Al.sub.2 O.sub.3 concentration because diluted alkalies must be added slowly to Alum in order to prevent simultaneous precipitation of aluminum hydroxide. Furthermore, in most instances, by-product losses are considerable. Solutions containing substances such as calcium or sodium sulphate or ammonium sulphate are the by-products that are usually generated when the processes described in the patents cited above are used. Typically, a by-product loss ranging from 20 to 30% by weight depending on basicities produced and sources of alkali used is almost unavoidable.
Mixing and filtration problems are also associated with most of the processes known in the art. For example, when lime is used as the alkali in the process, serious mixing and filtration problems can be encountered. Also, crytallisation problems will unavoidably be encountered when sodium sulphate is formed as a by-product.
Therefore, although the technical advantages of poly aluminum sulphate have been recognized for several years, there are still some serious problems associated with its preparation and handling. It appears that it would be highly desirable to provide a process that would minimize the losses that can be encountered through the production of by-products as well as problems associated with the separation of these by-products.